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Thanks for sending the details through, I thought I'd put the answer back here in case anyone else comes up against the same issue.
What you had done was correct, the only part that was missing is that when you use the payload option "Use Following for Query String/Body" it sets Content-Type to "application/x-www-form-urlencoded"
What you need to do is add to the header "Content-Type" and then your API needed "application/json".
What also tripped me over is if you add to the header "content-type" AKA all lower case, it will actually end up sending "Content-Type" and "content-type" which then throws the error you were getting around incorrect syntax.